Electrographic printers and copiers utilizing developer comprising toner, carrier, and other components use a developer mixing apparatus and related processes for mixing the developer and toner used during the printing process. The term “electrographic printer,” is intended to encompass electrophotographic printers and copiers that employ dry toner developed on an electrophotographic receiver element, as well as ionographic printers and copiers that do not rely upon an electrophotographic receiver. The electrographic apparatus often incorporates an electromagnetic brush station or similar development station, to develop the toner to a substrate (an imaging/photoconductive member bearing a latent image), after which the applied toner is transferred onto a sheet and fused thereon.
As is well known, a toner image may be formed on a photoconductor by the sequential steps of uniformly charging the photoconductor surface in a charging station using a corona charger, exposing the charged photoconductor to a pattern of light in an exposure station to form a latent electrostatic image, and toning the latent electrostatic image in a developer station to form a toner image on the photoconductor surface. The toner image may then be transferred in a transfer station directly to a receiver, e.g., a paper sheet, or it may first be transferred to an intermediate transfer member or ITM and subsequently transferred to the receiver. The toned receiver is then moved to a fusing station where the toner image is fused to the receiver by heat and/or pressure.
In electrostatographic copiers and printers, pigmented thermoplastic particles, commonly known as “toner,” are applied to latent electrostatic images to render such images visible. Often, the toner particles are mixed with and carried by somewhat larger particles of magnetic material. During the mixing process, the magnetic carrier particles serve to triboelectrically charge the toner particles to a polarity opposite that of the latent charge image. In use, the development mix is advanced, typically by magnetic forces, from a sump to a position in which it contacts the latent charge image. The relatively strong electrostatic forces associated with the charge image operate to strip the toner from the carrier, causing the toner to remain with the charge image. Thus, it will be appreciated that, as multiple charge images are developed in this manner, toner particles are continuously depleted from the mix and a fresh supply of toner must be dispensed from time-to-time in order to maintain a desired image density. Usually, the fresh toner is supplied from a toner supply bottle mounted upside-down, i.e., with its mouth facing downward, at one end of the image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth, and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the toner-metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the development mix.
It is well known that toner is a powdery substance that exhibits a considerable degree of cohesiveness and, hence, relatively poor flowability. Since the force of gravity alone does not usually suffice in causing toner to flow smoothly from the mouth of an inverted toner bottle, other supplemental techniques have been used to “coax” the toner from the bottle. For example, flow additives, such as silica and the like, have been added to the mix to reduce the troublesome cohesive forces between toner particles. See, e.g., the disclosure of U.S. Pat. No. 5,260,159 in which a “fluidization” agent is added to a developer mix in a development sump to assist the movement of developer therein. While beneficial to a more consistent flow of developer, such substances influence other performance attributes of the development process and their effectiveness is therefore constrained.
Development stations require replenishment of toner into the developer sump to replace toner that is deposited on the photoconductor or receiver as well as a magnetic carrier that are mixed together uniformly to form an effective developer. The developer must be mixed and transported to a position where it can be in contact with the latent charged image. If the mixing and/or transport are inefficient or ineffective the printing process is compromised. This can lead to many problems from poor prints to a no prints at all. In electrostatic development stations utilizing carrier, this is especially challenging since the magnetic carrier is affected by many conditions including particle size and orientation. Although the developer can stay near the feed roller at the front of the roller, as the developer with the feed roller encounters an increasing magnetic field is imposed on the developer is attracted away from the feed roller. As the feed apparatus picks up developer from the feed roller the amount of developer left near the rear portion of the feed roller is greatly decreased to the point where there is no developer left to transport to the latent charge image and printing stops. This is not an easy problem to solve since a simple change in developer amount or charge can quickly change conditions within the feed channel. This problem is enhanced since when there is less developer left in the feed channel then the pick-up point becomes even further from the feed roller and since the magnetic force is decreased by multiples as the distance decreases this makes the problem quite significant.
The present invention corrects the problem of non-uniform transport of developer from the feed roller to the feed apparatus. The apparatus and related methods described correct the problem of non-uniform developer feed in order to allow the printer to produce the high quality prints or powder coatings required by consumer demand. The following invention solves the current problems with developer feed rollers and will work in a wide variety of situations and with different types of toners, powders, or particles.